This invention relates to a braking system for automatically applying the brakes of a trailer when the trailer surges forwardly relative to a towing vehicle. More particularly, the invention pertains to an electrical surge braking system for a trailer having wheel brakes which are engaged when associated electrical coils are energized by the battery of the towing vehicle.
Typically, such a brake system includes a rheostat which is connected in a series path with the battery, the brake coils and the stoplight switch of the towing vehicle. The trailer usually is coupled to the towing vehicle by two hitch members which move relative to one another when the trailer tends to overrun and surge forwardly with respect to the towing vehicle during application of the brakes of the towing vehicle. The resistor of the rheostat is carried by one of the hitch members while the wiper of the rheostat is carried by the other hitch member.
When the brakes of the towing vehicle are applied and the trailer surges forwardly, an energizing path between the battery and the brake coils is established through the stoplight switch and the rheostat. As surge occurs, the wiper of the rheostat moves along the resistor and decreases the effective resistance value of the resistor in proportion to the relative displacement of the trailer. In this way, the energizing current to the brake coils is modulated to cause the braking torque to vary as a function of the displacement of the trailer.
The electrical connection between the battery of the towing vehicle and the rheostat of the trailer is established by tapping the stoplight circuit of the towing vehicle. In most modern vehicles made in the United States, the stoplights not only are energized when the brakes of the towing vehicle are applied and the stoplight switch is closed but also are flashed on and off when the hazard switch is actuated and repetitively closes and opens. The stoplight switch and the hazard switch usually are connected in parallel with one another in the stoplight circuit of the towing vehicle. Because that circuit is tapped to establish an electrical connection to the trailer, the rheostat and brake coils of a conventional surge braking system are repetitively pulsed with current whenever the hazard switch is actuated and the stoplight switch is open. In a conventional system, therefore, the trailer brakes will be repeatedly applied and released when the hazard switch is actuated even though there is no surge and no braking is required.
It has been proposed to eliminate the repetitive pulsing effected by the hazard switch by building a so-called "deadband" into the rheostat. For example, the portion of the resistor normally contacted by the wiper in the absence of surging may be insulated or made non-conductive so that, even if the hazard switch is closed, current will not be conducted to the brake coils by way of the rheostat until such time as surge occurs and the wiper moves off of the deadband or non-conductive portion of the resistor and onto the conductive portion thereof.
While the provision of a deadband in the rheostat overcomes the problem of repetitive braking caused by actuation of the hazard switch, the deadband introduces another problem. That is, once surge braking has been initiated, some trailers inherently tend to oscillate back and forth relative to the towing vehicle. During such oscillation, the wiper may repeatedly move onto and off of the deadband and may cause "on and off" braking of the trailer rather than smooth, continuous braking of the type which is effected when the brake coils are constantly energized. The on and off braking action sometimes accentuates oscillation of the trailer.